1. Field of the Invention
The present invention relates to a wheel for an automobile and a manufacturing method therefor which can lighten the wheel to a great extent without decreasing its fatigue strength.
2. Description of Related Art
As illustrated in FIGS. 8 and 9, an automobile wheel includes a disc 1 and a rim 2 connected to each other by spot welding 3. The spot welding 3 necessarily includes a stress concentrated portion 5 (FIG. 10) which tends to weaken the wheel. To compensate, the rim 2 and the disc 1 are engaged with each other axially over a relatively large length m' (FIG. 9). Clearance between the rim and the disc 1 is kept as small as possible because a tight engagement increases the strength of the wheel and decreases automobile vibration.
Disc 1 is manufactured, as generally illustrated by two dotted lines in FIG. 3, by die-cutting a substantially square flat plate having a side of length L' along a circle having a diameter D' greater than L', thereby obtaining a disc material portion 4. Disc material portion 4 is then formed into a disc shape to obtain a disc. In this instance, an arcuate portion 4a of the disc material portion 4 is formed into an axial end 4a of the disc flange shown in FIG. 9. Straight portion 4b of the disc material is formed into an arcuate recessed portion (or ventilation) 4b of the disc of FIG. 9. To increase the strength at the weld portion and to suppress radial displacements of the rim wall from a true circle, as described above, a most recessed portion 4c of the ventilation 4b is located axially inside a center of curvature 6 of the well radius between a rim well 8 and a rim side wall 9 so that a disc flange 7 contacts the rim well 8 at all circumferential portions of the disc flange 7. It has been thought from past experiences that if the most recessed portion 4c of the ventilation 4b were located axially outside the center of curvature 6 of the well radius, the strength of the welding portion would be greatly decreased and radial displacements of the rim from a true circle would increase. As a result, the length L' of the side of the disc material portion 4 is determined from the position of the most recessed portion of the ventilation when the disc 1 is pressed into the rim 2. More particularly, an axial length m' of 5 to 7 mm remains for engagement between the rim 2 and the disc 1 at the most recessed portion of the ventilation.
Despite that, there are two situations where the most recessed portion of a ventilation is located axially outside the center of well radius of a rim: one is where a periphery of the ventilation is bent to reinforce the ventilation, as illustrated in FIGS. 11 and 12, and another is where a specially shaped blank is used as a disc material as illustrated in FIGS. 13 and 14. These situations have particular problems described below.
More particularly, in the disc shown in FIGS. 11 and 12, the ventilation has an axially bent portion or rib G. Rib G is defined as a portion having a cross section of a different shape from that of a portion of the disc, other than the most recessed portion and located on the same radius as the most recessed portion with respect to a center of the disc. The bent portion G extends also circumferentially, and at each circumferential end the bent portion G of each ventilation has two curved portions having respective curvatures R.sub.1 and R.sub.2 opposite to each other. Because of this bent portion G: (1) A volume inside the wheel is reduced by .DELTA.R in radius so that the space available for mounting a brake is decreased. (2) Rigidity of the disc becomes large relative to that of the rim, so that relatively large radial displacements are caused in the rim, which lower the fatigue strength and life of the weld portion. Increased rigidity of the circumferential ends of the ventilation also increases rim deformation, which increases the radial displacements of the rim. (3) It is necessary to increase a size of the disc material portion in proportion to an axial length L.sub.1 of the bent portion.
Further, in the disc shown in FIGS. 13 and 14, since the disc material has an angularly cut portion K, the disk also has an angular portion K' in the ventilation which shortens the fatigue life of the wheel. Use of this type of disk in automobiles is therefore restricted.
However, in the wheel of the type generally shown in FIG. 9, the rim and the disc are frequently welded to each other by a weld which is continuous, at least locally, in a circumferential direction of the wheel. As a result, excess strength and rigidity of the wheel is created.